1. Field of Invention
Embodiments exemplarily described herein generally relate to methods of forming semiconductor devices and, more particularly, to a method of forming a semiconductor device in which gate insulating layers with different thicknesses are required.
2. Description of the Related Art
Semiconductor devices such as nonvolatile memory devices have a characteristic that it retains stored data even if power supply is interrupted. Currently, a flash memory device having a floating gate is a representative nonvolatile memory device which has been most well-known. In the flash memory device, according to whether or not charges are stored in the floating gate which is electrically isolated, a flash memory cell stores data of logic value “1” or “0”.
Typically, the flash memory device includes a cell region and a peripheral circuit region.
The peripheral circuit region includes a high voltage region where a high-voltage transistor is formed, and a low voltage region where a low-voltage transistor is formed. It may be required that gate oxide layers formed in these regions should be formed with different thicknesses to satisfy the characteristics of the respective regions.
It is known that thin gate oxide layers having the same thickness are formed in both the cell region and the low voltage region but a thick gate oxide layer is formed in the high voltage region. A flash memory device is conventionally formed by forming a thick gate oxide layer over an entire surface of a semiconductor substrate. Thereafter, the thick gate oxide layer over the cell and low voltage regions is removed using a photoresist pattern. Subsequently, a thin gate oxide layer is formed again over the cell and low voltage regions. Afterwards, a doped polysilicon layer is formed on the semiconductor substrate. Finally, a floating gate of the cell region, a gate electrode of the low voltage region, and a gate electrode of the high voltage region are formed from the doped polysilicon layer.
However, as the flash memory device has various characteristics and it becomes highly integrated, it may be difficult to reliably form different elements such as the flash memory cell, the low voltage transistor and the high voltage transistor, using the conventional method described above. For example, the photoresist layer is generally used for selectively removing the thick gate oxide layer of the cell and low voltage regions. Consequently, the thick gate oxide layer of the high voltage transistor can become contaminated with organic materials. Other problems may also be encountered during conventional processing. Therefore, a method capable of achieving various characteristics of the elements performing different functions is increasingly in demand.